The present invention relates in general to electrified vehicles such as hybrid electric vehicles, and, more specifically, to inverter-driven traction motors with reduced losses in the inverter power switching devices.
Electric vehicles, such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and full electric vehicles, use inverter-driven electric machines to provide traction torque and regenerative braking torque. A typical electric drive system includes a DC power source (such as a battery pack or a fuel cell) coupled by contactor switches to a variable voltage converter (VVC) to regulate a main bus voltage across a main DC linking capacitor. A first DC-to-AC inverter is connected between the main bus and a traction motor to propel the vehicle. The motor can be an induction motor or a permanent magnet motor, for example. A second DC-to-AC inverter is connected between the main bus and a generator to convert mechanical power from an internal combustion engine into electricity as a DC voltage for powering the traction motor (via the first inverter) or for recharging the battery. The second inverter may also be used to regenerate energy during braking to recharge the battery through the VVC.
The inverters each include transistor switches (such as insulated gate bipolar transistors, or IGBTs) connected in a bridge configuration. An electronic controller turns the switches on and off in order to invert a DC voltage from the bus to an AC voltage applied to the motor, or to invert an AC voltage from the generator to a DC voltage on the bus.
The inverter pulse-width modulates the DC link voltage to deliver an approximation of a sinusoidal current output to drive the traction motor with a desired speed and torque. The inverter outputs a series of pulse-width modulated (PWM) square wave voltages as a result of the coordinated switching of the IGBTs. The IGBTs and their reverse-recovery diodes have associated switching losses. In addition, the pulse-width modulation creates harmonic content losses in the motor.
In the circuit topology typically used in hybrid electric vehicle drives, the DC link is common to the generator inverter, battery converter, and traction motor inverter. Consequently, all three share the same substantially-constant DC voltage. The magnitude of the DC link voltage is generally chosen to provide the best operating efficiency for the generator and battery and to be sufficiently high to enable the motor inverter to achieve the upper end of the speed and torque ranges that are specified.
Switching losses in the traction inverter are higher with higher voltages across the inverter. A characteristic of traction motors is that they do not require as high of a voltage magnitude at low angular velocity operating points as they do at higher angular velocity operating points. In order to achieve the lower speed and torque when the DC supply across the inverter is constant, the duty cycle of the pulse-width modulated switching is decreased. Since the higher voltage required for higher speed/torque operating points still appears across the inverter, the same losses are incurred at the lower speed/torque operating points. Thus, during low speed vehicle operation there would be an opportunity to reduce the losses if the DC-link voltage present at the input of the inverter was reduced. Reducing the magnitude of the DC-link voltage present at the input to the inverter would reduce switching losses in the IGBTs, reverse recovery losses in the anti-parallel diodes, and harmonic losses in the traction motor. Reducing the energy losses would improve overall fuel economy.